(a) Field of the Invention
The present invention relates to a catalyst for reducing nitrogen oxides (which means N.sub.2 O, NO and NO.sub.2 in general) in flue gas or industrial waste gas coming from facilities such as boilers, heating furnaces and incinerators and a process for reducing nitrogen oxides to nitrogen with a reducing agent such as ammonia in the presence of the catalyst.
(b) Description of the Prior Art
For the removal of nitrogen oxides present in a waste gas, there have been proposed various processes. These processes comprise the catalytic oxidation process, the catalytic decomposition process, the catalytic reduction process and the adsorption process. The catalytic reduction processes are classified into the selective reduction process and the non selective reduction process according to whether the reducing agent selectively reacts with nitrogen oxides in the waste gas or not.
In the selective reduction process, hydrogen sulfide, ammonia, etc. are ordinarily used as the reducing agent. A noble metal catalyst containing such as Pt metal and a metal oxide catalyst containing an oxide or oxides of metal such as copper and iron are well-known as the selective reducing catalysts of nitrogen oxides in the presence of ammonia. A complex catalyst comprising two or more metal oxides may be used.
In an industrial waste gas which contains nitrogen oxides usually together with 1,000 to 2,000 ppm of sulfur oxides (which means SO.sub.2, SO.sub.3, etc. in general), conventional catalysts, particularly noble metal catalyst is degraded in activity during the catalytic reducing reaction of nitrogen oxidos, because of the poisonous effect of sulfur oxides, and hence the continuous operation for a long period of time is difficult to be performed. Thus, this process can not be commercially applied for reducing nitrogen oxides in the waste gas containing sulfur oxides. Therefore, in the operation of the catalytic NO.sub.x -reducing process in the industrial scale, it will be necessary to remove sulfur oxides from the waste gas beforehand or to use catalysts having high resistivity to the poisonous effect of sulfur oxides.